The effect of uniaxial pressures on the infrared spectra of LiNH$_4$SO$_4$ crystals

The infrared reflection spectra of mechanically free and uniaxially compressed LiNH$_4$SO$_4$ crystals are studied for the first time in the spectral range of 800–1700 сm$^{-1}$ along three crystallophysical directions. The Kramers–Kronig dispersion relations are used to determine the dispersion and baric dependences of refractive index n and the real $\varepsilon_1$ and imaginary $\varepsilon_2$ parts of the dielectric constant and to calculate the frequencies of longitudinal $\omega_{\mathrm{LO}}$ and transverse $\omega_{\mathrm{TO}}$ vibrations, decay constant $\gamma$, and oscillator strength f of mechanically free and compressed LiNH$_4$SO$_4$ crystals. The considerable changes observed in the main reflection bands are explained by the effect of uniaxial pressures on the NH$_4$ and SO$_4$ tetrahedra.